1. Field of the Invention
The present invention relates to an image processing device and an image processing method and, in particular, relates to an image processing device and an image processing method of obtaining a higher quality printed matter while controlling the amount of color materials applied such as toner or ink.
2. Description of the Related Art
There are increasing chances to print data created by e.g., a personal computer using an image processing device having a printing function. Since the amount of consumption of color materials drastically affects running costs, there has conventionally been known an image processing device having the function intending to reduce the amount of consumption of color materials to be used. General methods of implementing a saving function of such color materials include the method of reducing the density value of each color corresponding to each color material of image data before printing.
In an electrophotographic image processing device typified by a laser printer of laser beam exposure type, although image formation is performed by adhering a toner, there are present various types of materials to be used for printing. On this occasion, in some cases, depending on the type of material, toner does not fix fully and toner is splashed, or toner comes out at the portion of high dot density. As measures to cope with these problems, the method of reducing the amount of toner adhesion is taken. The implementation method thereof includes the one that reduces the density value or the gradation value of each color corresponding to each color material of image data before printing.
In addition, recently not only reduction in the amount of consumption of color materials or effects on the material as mentioned above, but also noticed are effects of decrease of a fixing temperature of a fixing unit by reducing the amount of color materials applied per each pixel. Due to this decreased fixing temperature, effects of power saving of the fixing unit, downsizing, and shortening of a first copy-out time are expected.
However, in case where the amount of color materials applied is decreased, there is a possibility of image deterioration as compared with the case of the normal applied amount. Specifically, there is a possibility of reduced reproduction capability due to narrowed color regions able to be reproduced. Furthermore, contour portions of e.g., such characters or graphics as to be at the maximum density at the time of the normal applied amount come to be of halftone colors owing to the decrease in density of image data. As a result of halftone processing, there is a possibility of the occurrence of jaggy (stepped jaggy that appears on slanting lines) or discontinuity. Conventionally, since at the time of setting a saving function of color materials, proof or draft printing is assumed, an image quality tends not to be regarded as important. In recent years, however, various effects as described above have been known, and high quality on the occasion of reducing the amount of color materials applied is expected.
As a countermeasure against image deterioration (jaggy, discontinuity or the like) on such occasion of controlling the amount of color materials applied, conventionally proposed is the one that an edge determination is made with respect to character portions or graphic portions, and specific processing such as contour enhancement and pattern replacement is performed only with respect to the edge portions (Japanese Patent Laid-Open No. 11-55519 and Japanese Patent Laid-Open No. 11-187270). Furthermore, generally as a countermeasure against jaggy or discontinuity at the edge portion illustrated with halftone colors, increase of the density of a halftone dot at the time of image formation, that is higher screen ruling has been known. Normally, the type of object to be printed is included in a print data as information. Thus, changes in a screen ruling depending on the type of object, that is the method of applying a high screen ruling to e.g., characters and graphics, and on the other hand, applying a low screen ruling with respect to a nature image has been conducted. As to a screen ruling, in the case of printing an image, fine halftone dots arranged on a contact screen film is overlapped on the image, and by receiving lights passing through the halftone dots, shading of the image is converted to the size and the density of the halftone dots. Here, the halftone dots are provided at the high density with increasing proximity to the center. Bright lights are extracted as small dots and dark lights are extracted as large dots. The screen ruling component how many lines of halftone dots are present in the width of about 25.4 mm (1 inch), and the unit is indicated with lpi (line per inch). In general, when using a high screen ruling, the halftone dots come to be unobstructive with naked eyes and high quality printed matters can be obtained; whereas, when using a low screen ruling, the halftone dots become marked and thus printed matters show rough appearance. However, in electrophotographic image processing devices, since electrophotographic processes are unstable when using high screen ruling, a problem exists in the occurrence of discontinuity and instability of density characteristics. To solve these problems, for example, in Japanese Patent Laid-Open No. 2002-223357, proposed is the one of switching a screen ruling in accordance with an output density value. Here, in the low-density region, the screen ruling is decreased, and output characteristics of the electrophotographic process are made stable, while in the high density region, the high screen ruling is applied, thus enabling a high-resolution and sharp output.
However, conventional countermeasures against jaggy or discontinuity on the occasion of controlling the amount of color materials applied requires the function of edge determination. Here, in case of implementing the edge determination function, line buffer and the like for use in pattern matching comes to be necessary, thus to be costly. Furthermore, the edge determination may raise error determination, and this error determination may result in image deterioration.
On the other hand, increase of the screen ruling, to keep stability of the electrophotographic process, irrespective of the king of objects, is desirably performed selectively based on image data. However, in the conventional method of switching screens in accordance with a density value, in the case in which the amount of color materials applied is controlled, since the density value is varied by control of the applied amount, it is difficult to make appropriate switching of screens.